Well, just supposing you did decide to read the article...
You'd find out that the gambling ordinance exempts penny-ante games with piddling stakes. In fact, it specifically cites mah-jong as a non-crime.
But, you'd only know that if you read the article.
Do you really think the person that complained said there was a perfectly legal game taking place? Don’t you think the police have and obligation to investigate a report of illegal activity taking place? I know it is easy to make decisions after all the facts have been obtained, but my guess is, they didn’t have the facts you do now.